The present invention relates to a method for desulfating an NOx accumulator or an NOx accumulator catalytic converter installed downstream of an internal combustion engine.
NOx accumulator catalytic converters are used to store NOx in the exhaust of internal combustion engines operated with a lean mixture. However, with sulfur-containing fuels of the type that are currently customary used, stable sulfates form on the catalytically active surfaces, leading to a gradual poisoning of the catalytic converter, gradually reducing its NOx storage ability. However, in contrast to lead poisoning of 3-way catalytic converters, such sulfate poisoning of an NOx accumulator catalytic converter is completely or at least largely reversible by operation at a sufficiently high catalytic converter temperatures of over approximately 550xc2x0 C. with a sufficiently high pollutant supply and a low residual oxygen content.
Consequently, in practice NOx accumulator catalytic converters are periodically desulfurized or desulfated by setting a suitable temperature and reducing the oxygen content of the exhaust stream, in other words lowering the lambda value. In this context, the lambda value is generally below 1.05 and preferably even below 1, in the rich operating region of the internal combustion engine, since desulfating at higher lambda values would take place at too low a reaction rate, which would entail undesirably long desulfating times.
During desulfating of NOx accumulator catalytic converters, the stored sulfur is released primarily in the form of sulfur dioxide (SO2) and hydrogen sulfide (H2S), in which process other sulfur-containing molecules are also formed in small quantities. With richer exhaust gas, in other words with lowered lambda values, less SO2 is produced and more H2S is produced. Production of H2S, however, is undesirable on account of its unpleasant odor. In order to avoid such an unpleasant odor, the desulfating of NOx accumulator catalytic converters in the current state of the art takes place at lambda values of more than approximately 0.98, since H2S is then produced in only negligible quantities. This advantage is, however, still associated with quite long desulfating times and correspondingly higher fuel consumption.
The object of the present invention is thus to create a process that makes it possible to desulfate an NOx accumulator catalytic converter in the shortest possible time while suppressing the formation of undesirable byproducts.
In accordance with the invention, during desulfating the internal combustion engine is operated in a plurality of rich/lean cycles with a cyclic reduction of the lambda value to less than 0.98 during the rich operation. In order to avoid an unpleasant odor resulting from hydrogen sulfide, the duration of the rich operation is chosen to be so short that, little or no noticeable emission of hydrogen sulfide takes place.
The method according to the invention is based on the discovery that, when the lambda value is lowered to below approximately 0.98, the sulfur stored in the catalytic converter is initially released at an accelerated rate in the form of a characteristic SO2 emission peak, while hydrogen sulfide formation begins only after a certain time delay, occurs significantly more slowly, and continues over a longer period of time than SO2 formation.
Therefore, during desulfating of a poisoned NOx accumulator catalytic converter, after a predetermined desulfating temperature is established a majority of the stored sulfur is initially released by establishing a lambda value of less than 0.98, while the undesirable formation of hydrogen sulfide may be deliberately prevented by a timely return to a lean operating condition. After the end of such a rich/lean cycle there follows one or more further cycles in order to release the remaining sulfur and regenerate the catalytic converter to the desired degree of desulfurization.
Through this process, in accordance with the invention, not only can the formation of undesirable byproducts, and especially the formation of hydrogen sulfide, be effectually suppressed, but the result is significantly shorter regeneration times than with the state of the art, in spite of the use of multiple regeneration cycles, since the individual cycles are short, because of the strong acceleration of SO2 formation with decreasing lambda value.
In the process in accordance with the invention, lambda values between 0.88 and 0.985 prove to be especially advantageous. The region between 0.93 and 0.96 is to be preferred in this regard, while values between 0.945 and 0.955 prove to be especially beneficial.
The duration of rich mixture operation is preferably between 0.5 and 30 sec, while the range between 1 and 20 sec, and in turn especially the range between 2 and 10 sec, proves especially advantageous.
The lean periods of the rich/lean cycles associated with these reduction periods or rich periods are preferably between 0.5 s and 10 s, wherein the range between 2 s and 6 s proves especially advantageous.
The choice of the optimal process parameters, as for example the rich and lean periods, the number of rich/lean cycles and the lambda values used in each case, is primarily guided by the noble metal and storage substances used in each case, the O2 storage capacity of the wash coat, the catalytic converter volume, the space velocity, the composition of lean and rich exhaust gases, the mass of stored sulfur, the spatial distribution of the stored sulfur and the temperature of the storage catalytic converter. With regard to optimizing the process it can also prove advantageous herein for the rich and lean periods and/or the lambda values in the individual rich/lean cycles to be chosen differently.
Additional features and advantages of the process in accordance with the invention result not only from the associated claimsxe2x80x94alone and/or in combinationxe2x80x94but also from the following detailed description of a preferred example embodiment in conjunction with the associated drawings.